drm/panfrost: Shrink sched_lock

drm/scheduler requires a lock between _init and _push_job, but the
reservation lock dance doesn't. So shrink the critical section a
notch.

v2: Lucas pointed out how this should really work, I got it all wrong
in v1.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Steven Price <steven.price@arm.com>
Cc: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210622165511.3169559-5-daniel.vetter@ffwll.ch